Method and apparatus for the joint design and operation of arq protocols with user scheduling for use with multiuser mimo in the downlink of wireless systems

ABSTRACT

A method and apparatus is disclosed herein for performing wireless communication. In one embodiment, the apparatus comprises a processing unit to run a scheduling selection algorithm to update user terminal scheduling weights in response to scheduling feedback transmitted by a plurality of user terminals by an end of an immediately preceding scheduling event; a scheduler and precoder, responsive to the updated user terminal scheduling weights generated by the scheduling algorithm and channel estimates of user terminals, to choose a set of user terminals for scheduling and to choose precoder beams and their power for such user terminal in the set of user terminals; a plurality of precoding blocks to receive one coded ARQ block for at least one packet for each user terminal in the set and, responsive to the precoder beams, to generate precoded data, where the one coded ARQ block is one of a plurality of ARQ blocks generated for a single packet and being generated using a single ARQ scheme for such each user terminal; and a transmitter to transmit the precoded data using MIMO transmission.

PRIORITY

The present patent application is a divisional of and incorporates byreference U.S. patent application Ser. No. 12/862,651, titled “Methodand Apparatus for the Joint Design and Operation of ARQ Protocols withUser Scheduling for Use with Multiuser MIMO in the Downlink of WirelessSystems,” filed Aug. 24, 2010, which claims priority to and incorporatesby reference the corresponding provisional patent application Ser. No.61/237,046, titled, “Method and Apparatus for The Joint Design andOperation of ARQ Protocols with User Scheduling for Use with MultiuserMIMO in the Downlink of Wireless Systems,” filed on Aug. 26, 2009.

FIELD OF THE INVENTION

Embodiments of the present invention relate to the field of use ofuser-scheduling and automatic repeat-request (ARQ) protocols (errorrecovery mechanisms) for delivering information bearing signals touser-terminals (UTs) in the downlink of wireless cellular systems; moreparticularly, the present invention relates to joint use ofuser-scheduling and ARQ protocols with multiuser (MU) multiple in,multiple out (MIMO) for delivering information bearing signals touser-terminals (UTs) in the downlink of wireless cellular systems.

BACKGROUND OF THE INVENTION

In communication systems, the transmission system has to make decisionsabout which users to schedule, and with what rates. Such decisions aremade by a scheduling algorithm within the system, and transmissions inline with such decisions are formed by the multi-user transmissionsystem, e.g. by choice of the MU-MIMO signaling.

However, a major problem in most existing systems is the fact that themaximum possible rate that can be delivered to the user in eachscheduling cycle is often not known exactly. For example, in a wirelesssystem the signal, noise and interference levels as seen by each user ina scheduling cycle are all important in being able to determine the ratethat can be reliably sent to this receiving user. Unfortunately in manycellular systems such levels are not known exactly at the transmittingparty at the time of transmission.

As an example, the level of inter-cell or inter-cluster interference(ICI) level varies from scheduling cycle to scheduling cycle and isoften unknown for at least some scheduling cycles. Furthermore, ICIlevels can often be greater than the useful-signal level for many users.Unknown ICI and other sources of uncertainty in the system related topredicting user rates can have a significant effect on the efficiency ofboth the scheduling as well as MU-MIMO downlink signaling schemes sincesuch schemes depend on such predictions.

The uncertainty in the rate the channel can support by naturenecessitates the use of an error recovery mechanism, such as ARQ. Thisis because the transmitted rate to a user may not be supportable by theinstantaneous channel a user is experiencing.

As further background, it should be noted that user-scheduling, ARQ andMU-MIMO methods are used in many state of the art multi-user systems,and are individually well known to those familiar with the state of theart.

In many state of the art systems each base-station cluster controller(or base-station) has a number of UTs to serve. For each schedulingcycle, the scheduler in the system chooses the UTs to schedule and thepower and rate allocation to such UTs. Such decisions are often based onestimates of the instantaneous channels, or quality of instantaneouschannels, between the cluster and its UTs. Exploiting the variation insuch channels, e.g. scheduling users in scheduling cycles when suchusers have very good channels, can improve overall system performance.This variation is thus exploited by the scheduler.

In trivial systems, e.g. those that simply maximize per cell throughput,the criterion directing the operation of the scheduler can often lead toa case where users are not served fairly. For example, for a criterionto maximize per cell throughput, it often makes sense to serve usersnear base-stations, starving as a result those users at cell edges.Thus, user scheduling is also an essential component in many systems tonot only increase performance by exploiting channel variations, but toalso ensure user fairness with respect to a chosen criterion.

There are many such scheduling methods to do so, known to those familiarwith the state of the art. One such example is a “Proportionally FairScheduler (PFS)” which tries to maximize the geometric mean of the peruser average throughput. Another is a “MaxMin” scheduler which tries tomaximize the minimum per user average rate. Many schedulers, PFS andMaxMIn being two such examples, can be implemented by maximizing aweighted sum rate criterion. The weights are linked intimately to thecriterion. In one commonly used scheduling weight generation mechanismfor PFS, the weights are set as the inverse of the average throughput ofeach user, which is estimated in a loop with the physical layerparameters. An alternative method for generating the user-specificscheduling weights (and can be used for both MaxMin and PFS as well as avery broad range of other scheduling criteria) relies on the use ofvirtual queues. These procedures are well known to those familiar withthe state of the art.

Effective operation of such schedulers depends often on being able topredict what rates can be delivered in each scheduling cycle to eachuser, if the user was scheduled in the scheduling cycle. The rates thatcan be delivered in each case depend on knowing the quality of theoverall instantaneous channel at each UT. There are many well knownmethods to do so. The rate depends on, among other things, the power orcovariance of the combined interference caused by all neighboringcluster transmissions. As noted earlier, this interference is referredto as intercluster interference (ICI).

The ICI power level experienced by a user depends on the instantaneouschannels between the UT and neighboring cluster antennas, and theMU-MIMO precoded signals transmitted by these clusters. As a result, inprinciple, the instantaneous ICI experienced by each UT is not known toits cluster at the time of scheduling and transmission. This is because,by definition, clusters do not fully coordinate.

Other related uncertainties in quantities, such as partialchannel-state-information (CSI) regarding the channel between each UTand its cluster, yield similar uncertainties in determining thetransmission rates that can be supported by each UT instantaneouschannel.

By definition, the uncertainties in the predicted user rates lead tolosses in performance since the scheduler is not able to make theperfect decisions for each scheduling cycle. For example, a transmissionto a user may fail if the transmission rate to the user is set higherthan what the instantaneous channel can support. In such cases, outagesin transmissions occur and the system can waste all or part of theresource that was available in the scheduling cycle.

One way to reduce such losses is to reduce the probability of suchoutages. A simple way to do so is to lower the transmission rate tousers such that the probability of such an outage event is at anacceptable level. However, this often means the system operatesconservatively and well below the potential throughput it can deliver.This is particularly true for edge users whose average possible rate ina scheduling cycle is often far less than larger instantaneous ratesthey could obtain when channels to such users are known and areinstantaneously favorable.

Another way is to design the system such that such uncertainties areminimized. Reducing uncertainty can for example be achieved bytechniques such as:

Increasing the frequency reuse factor, which lowers ICI levels so thattheir effect, even with variation has a small effect on rate. However,increased reuse factors means less available resources per cluster (orcell) for transmission, and often an overall loss in performance.

Reducing the variation in scheduling and transmission signals fromscheduling cycle to scheduling cycle so that clusters can estimate ICI.This demonstrates the interplay between scheduling and losses. However,since such methods constrain the scheduler, system efficiency can bereduced. These methods also do not address, directly, the issue of howto link the error recovery mechanism to the scheduling, except to say itmakes the job of the scheduler somewhat easier (but not easy).

Reduction in uncertainties can reduce inefficiencies. However, onceuncertainties exist about potential deliverable rates, outage basedapproaches will not be able to fully use all available transmissionresources as failed transmissions represent unused transmissionopportunities.

In contrast to such methods, one can consider ARQ methods. These are aclass of methods suited for settings where outage rates are high. Suchmethods are based on exploiting successful decoding acknowledgementssent by each UT to its cluster via a low-rate feedback channel. Oneparticular class of such methods are known as Hybrid ARQ methods. Thesenot only exploit feedback but are also able to reuse past unsuccessfultransmissions to help in the decoding of future transmissions. In thisway, the waste from “outages” is minimized.

For example, take the case where a single user is served within asequence of scheduling cycles and the instantaneous deliverable rate tothe user fluctuates within the scheduling interval. If each schedulingcycle transmission is independent then the system operates with outagewhere some transmissions are often lost and some possibly received.

A Hybrid-ARQ (H-ARQ) scheme has dependencies between scheduling cycletransmissions such transmissions help each other. For example, thosescheduling cycles experiencing times of low possible delivered rates canbe helped by other transmissions where channels are more favorable. Infact, a properly designed H-ARQ system can deliver rates close to themaximum sum throughput (the sum of all instantaneous deliverable ratesover all scheduling cycles) that can be supported by the channel overthe scheduling interval, albeit at a cost of decoding delay for variousinformation bits. In general, the higher the decoding delay that can betolerated by the user (application), the closer the delivered userthroughput can be made to its maximum possible level. In the context ofa user-specific (e.g., application-based) decoding delay constraint, onewould want to have the H-ARQ system designed (i.e., its parametersoptimized) in such a way that it maximizes the rate delivered to theuser among all systems that do not violate the decoding delayconstraint.

In the systems of interest, however, involving MU-MIMO transmission,with scheduling criteria, such as proportional fair scheduling (PFS), asystematic design and optimization of the UT-ARQ parameters isnontrivial. Indeed, varying the ARQ parameters of a given UT affects therates/decoding delay trade-offs of the given UT but also those of allother UTs in the cluster. This is because a change in the ARQ parametersof a UT also affects the scheduling algorithm (e.g., the PFS weights incase a PFS criterion is used). Therefore, ARQ parameters affect therates and users scheduled, the effective system throughput, schedulingactivity fraction, decoding delays, etc. of all UTs in the cluster.

SUMMARY OF THE INVENTION

A method and apparatus is disclosed herein for performing wirelesscommunication. In one embodiment, the apparatus comprises a processingunit to run a scheduling selection algorithm to update user terminalscheduling weights in response to scheduling feedback transmitted by aplurality of user terminals by an end of an immediately precedingscheduling event; a scheduler and precoder, responsive to the updateduser terminal scheduling weights generated by the scheduling algorithmand channel estimates of user terminals, to choose a set of userterminals for scheduling and to choose precoder beams and their powerfor such user terminal in the set of user terminals; a plurality ofprecoding blocks to receive one coded ARQ block for at least one packetfor each user terminal in the set and, responsive to the precoder beams,to generate precoded data, where the one coded ARQ block is one of aplurality of ARQ blocks generated for a single packet and beinggenerated using a single ARQ scheme for such each user terminal; and atransmitter to transmit the precoded data using MIMO transmission.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detaileddescription given below and from the accompanying drawings of variousembodiments of the invention, which, however, should not be taken tolimit the invention to the specific embodiments, but are for explanationand understanding only.

FIG. 1 is a flow diagram of one embodiment of a process for transmittingdata in a wireless communication system.

FIG. 2 illustrates one embodiment of the ARQ block generation processthat depicts how information bits destined for a user terminal arepartitioned into information packets for an ARQ process and are used togenerate ARQ-coded blocks.

FIG. 3 illustrates one embodiment of scheduler state updating.

FIG. 4 illustrates a joint scheduler/precoder.

FIG. 5 illustrates one embodiment of an ARQ block selection process.

FIG. 6 illustrates the ARQ/PHY transmitter operation at the basestation.

FIG. 7 illustrates the processing that occurs at a user terminal.

FIG. 8 is a block diagram of one embodiment of a user terminal.

FIG. 9 is a block diagram of a computer system.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Embodiments of the invention focus on the use of automaticrepeat-request (ARQ) protocols in combination with a user-schedulingmethod for delivering information bearing signals to user-terminals(UTs) in the downlink of wireless cellular systems. Embodiments of theinvention are applicable to settings involving clusters of sets oftransmit antennas (collocated or not) simultaneously transmitting to UTsin their coverage area. Furthermore, these embodiments apply to the casewhere within each transmission cycle or instance, each cluster schedulesa subset of UTs for transmission and generates multiple spatial streamsthat are superimposed and simultaneously transmitted from multipleantennas to such UTs. In one embodiment, the transmission methodincludes what is referred to as multi-user multiple-inputmultiple-output (MU-MIMO) transmission.

The ARQ block generation process can be performed independently of thescheduling of user terminals. That is, the ARQ scheme can be selectedand ARQ blocks created for a user terminal independently of whether thatUT is scheduled. The sufficient abstraction of the ARQ process from thescheduling leads to what is referred to herein as a decoupled system.

In the following description, numerous details are set forth to providea more thorough explanation of the present invention. It will beapparent, however, to one skilled in the art, that the present inventionmay be practiced without these specific details. In other instances,well-known structures and devices are shown in block diagram form,rather than in detail, in order to avoid obscuring the presentinvention.

Some portions of the detailed descriptions which follow are presented interms of algorithms and symbolic representations of operations on databits within a computer memory. These algorithmic descriptions andrepresentations are the means used by those skilled in the dataprocessing arts to most effectively convey the substance of their workto others skilled in the art. An algorithm is here, and generally,conceived to be a self-consistent sequence of steps leading to a desiredresult. The steps are those requiring physical manipulations of physicalquantities. Usually, though not necessarily, these quantities take theform of electrical or magnetic signals capable of being stored,transferred, combined, compared, and otherwise manipulated. It hasproven convenient at times, principally for reasons of common usage, torefer to these signals as bits, values, elements, symbols, characters,terms, numbers, or the like.

It should be borne in mind, however, that all of these and similar termsare to be associated with the appropriate physical quantities and aremerely convenient labels applied to these quantities. Unlessspecifically stated otherwise as apparent from the following discussion,it is appreciated that throughout the description, discussions utilizingterms such as “processing” or “computing” or “calculating” or“determining” or “displaying” or the like, refer to the action andprocesses of a computer system, or similar electronic computing device,that manipulates and transforms data represented as physical(electronic) quantities within the computer system's registers andmemories into other data similarly represented as physical quantitieswithin the computer system memories or registers or other suchinformation storage, transmission or display devices.

The present invention also relates to apparatus for performing theoperations herein. This apparatus may be specially constructed for therequired purposes, or it may comprise a general purpose computerselectively activated or reconfigured by a computer program stored inthe computer. Such a computer program may be stored in a computerreadable storage medium, such as, but is not limited to, any type ofdisk including floppy disks, optical disks, CD-ROMs, andmagnetic-optical disks, read-only memories (ROMs), random accessmemories (RAMs), EPROMs, EEPROMs, magnetic or optical cards, or any typeof media suitable for storing electronic instructions, and each coupledto a computer system bus.

The algorithms and displays presented herein are not inherently relatedto any particular computer or other apparatus. Various general purposesystems may be used with programs in accordance with the teachingsherein, or it may prove convenient to construct more specializedapparatus to perform the required method steps. The required structurefor a variety of these systems will appear from the description below.In addition, the present invention is not described with reference toany particular programming language. It will be appreciated that avariety of programming languages may be used to implement the teachingsof the invention as described herein.

A machine-readable medium includes any mechanism for storing ortransmitting information in a form readable by a machine (e.g., acomputer). For example, a machine-readable medium includes read onlymemory (“ROM”); random access memory (“RAM”); magnetic disk storagemedia; optical storage media; flash memory devices; etc.

Overview

Techniques described herein combine the use of ARQ protocols withscheduling in a decoupled manner in which the ARQ parameters for each UTto achieve a desired fraction of the maximum (assumed) instantaneousUT-specific throughput given a delay constraint are decoupled from thescheduling decision of whether to schedule a particular UT.

FIG. 1 is a flow diagram of one embodiment of a process for transmittingdata in a wireless communication system. The process may be performed bya processing logic which may comprise hardware, software, or acombination. The following will be described in terms of the processinglogic of a base station; however, the processing logic may be part ofanother station, such as an access point in a wireless communicationsystem.

Referring to FIG. 1, the process begins by processing logic generatingmultiple ARQ blocks for a single packet that is to be sent to one of agroup of multiple user terminals (processing block 101). The multipleARQ blocks comprise a sequence of ARQ blocks to be transmitted in asequential order. Each of the ARQ blocks comprises coded samples. In oneembodiment, the multiple ARQ blocks are generated independently ofscheduling and precoding. In one embodiment, the multiple ARQ blocks aregenerated using a set of parameters associated with the user terminal,and the set of parameters are independent of the knowledge of theinstantaneous channel between the base station's transmitter and theuser terminal. In another embodiment, the ARQ blocks are generated byreceiving information bits that are destined for the user terminal,splitting the received information bits into multiple packets based onan ARQ block coding rate, and applying an ARQ block generator process toeach of the packets based on a signaled ARQ scheme and coding andmodulation parameters to produce the multiple ARQ blocks for each of thepackets. In one embodiment, the ARQ block generator process along withall its parameters, including the ARQ block coding rate and the codingand modulation parameters are set individually for each user, andindependently of the instantaneous scheduler/transmission operation andthe instantaneous channel conditions. In particular, these parametersare set/optimized taking into account the user-channel qualitydistribution across time, in the form, e.g., of a distribution of userICI power level or user SINRs (signal to interference plus noiseratios). These quantities, however, are not instantaneous but ratherestimates of ensembles of channel quality. Such quantities can beestimated and fed back by each user over long time scales, or can bealso estimated at the cluster controller over time based on informationfed back by each user. In general, how the ARQ parameters are set foreach user are also dictated by user application constraints. In oneembodiment, a user's ARQ mechanism parameters are set to optimize thedelivered rate subject to an average delay constraint. For example,given a set of possible configurations, the rate/delay trade-offs ofeach configuration would be estimated for the given user qualitydistribution profile, and among the configurations not violating theapplication delay constraint, the one yielding the highest long termthroughput would be selected.

Subsequent to generating the ARQ blocks for each single packet,processing logic updates user terminal scheduling weights in response toscheduling feedback transmitted by the user terminals by the end of theimmediately preceding scheduling event (processing block 102). The endof the immediate proceeding scheduling event occurs at the end of thecurrent transmission cycle. That is, at the end of the currenttransmission cycle, the user terminals transmit scheduling feedbackinformation to be used in the next scheduling event by the base station.

After updating the user terminal scheduling weights (and moving to thenext scheduling cycle), processing logic schedules a set of userterminals for the next transmission scheduling slot based on the updateduser terminal scheduling weights (processing block 103) and selectsprecoder beams and power levels for each user terminal that has beenselected for scheduling (processing block 104).

Next, processing logic selects for transmission one ARQ block from themultiple ARQ blocks generated for the one packet destined for the userterminal (processing block 105). In one embodiment, the selection of theARQ block is performed by determining whether the one user terminal isscheduled, and if the one user terminal is scheduled, determining iffeedback information, from the one user terminal, corresponding to thepreviously sent ARQ block for the packet is an acknowledgement (ACK)signal and selecting an ARQ block for another packet for transmission ifthe feedback information is an ACK signal or selecting another ARQ blockof the multiple ARQ blocks for transmission if the feedback informationis a negative acknowledgement (NACK) signal, where this ARQ block is thenext to be transmitted in the sequential order of the ARQ blocks for thepacket.

After selection of the one ARQ block for transmission, processing logicgenerates precoded data using a precoded beam and power selected for theone user terminal for the scheduling slot (processing block 106).Thereafter, processing logic transmits the coding precoded data(processing block 107). In one embodiment, the transmission of precodeddata occurs as part of a MIMO transmission process. Transmission of theprecoded data occurs such that the ARQ block for the packet arescheduled and transmitted over distinct transmission slots. In oneembodiment, the transmission of precoded data includes summing multiplesets of precoded data including the precoded data generated for the oneARQ block to produce the summed output, applying OFDM to the summedoutput, and transmitting the results of applying the OFDM to the summedoutput using an RF front-end.

ARQ Block Generation Process

FIG. 2 illustrates one embodiment of the ARQ block generation processthat depicts how information bits destined for a user terminal arepartitioned into information packets for an ARQ process and are used togenerate ARQ-coded blocks. The ARQ block generation process is performedby an ARQ block generation unit. This unit may be part of, orimplemented by, a processor or control logic for the transmitter usinghardware, software, or both.

Referring to FIG. 2, information bits for user terminal “m” comprise . .. , b_(t+1)(m), b_(t)(m), b_(t−1)(m), . . . , are received and splitinto packets (202). In one embodiment, the split is based on the ARQfirst-block coding rate 210. Once the parameters of the ARQ mechanismfor user “m” have been defined, then these are all set; this iseffectively dictated by the parameters of the selected ARQconfiguration, chosen e.g., among all available ones subject to the rateor delay constraints for the given user. In one embodiment, thisselection occurs on a user per user basis and is not varied from instantto instant. The packets produced as a result of the split are shown as .. . , info packets 1, . . . , info packet n−1, info packet n, and infopacket n+1, . . . . Each of these packets are input into an ARQ blockgeneration unit 203 that includes ARQ block generation sub-units 203 ₁through 203 _(n+1). In one embodiment, each of the ARQ block generationsis user specific.

The ARQ block generation sub-units 203 ₁-203 _(n+1) generate ARQ blocksin response to the ARQ first block coding rate 210, ARQ scheme 211 andcoding modulation parameters 212. In one embodiment, these parametersdepend on channel quality “distribution” seen by the individual userterminal as well as the user traffic and application demands andconstraints, but not the instantaneous channels that are measured andnot the instantaneous precoders that are used. As mentioned above ingeneral there is a data base of possible ARQ protocols, one per “choice”of parameters and the “choice” of parameters could comprise, e.g., thechoice of modulation scheme, outer code type and memory, outer coderate, and possibly, other parameters. This is well known in the art.Note that in one embodiment, these coding and modulation parameters areset in general in a user-specific manner and are not changedinstantaneously but rather over much longer time scales (e.g., as theuser location changes or the user demands or decoding constraintsdictated by the application change).

The outputs of the ARQ generation sub-units 203 ₁-203 _(n+1) are the ARQblocks, which represent blocks of coded ARQ symbols for various packets.These are coded complex-valued samples. For example, information packetn is input into ARQ block generation subunit 203 _(n) to produce ARQblocks (n,1), ARQ block (n,2), . . . , ARQ block (n, k_(max)). ARQ block(n, 2) represents the second block of coded ARQ symbols for packet n.

One key aspect of the techniques described herein is that the ARQ blockcoding rate and the associated ARQ coding mechanism parameters for anygiven user can be selected independently of all the other users: theycan be selected based on the user channel quality distribution (whichcan be estimated periodically), together with either: (1) the user'saverage decoding delay constraints (in that case the parameters areselected to maximize the average rate delivered to the user subject tonot violating the user's decoding delay constraints), or (2) the user's“optimistic” rate fraction requirements (e.g., achieve a 95% of theoptimistic rate, i.e., of the best throughput predicted by theory in the“genie aided” case where the BS knows a priori the instantaneous ratesupported by the channel in each instance for this user andinstantaneously varies the code to match it); in the second case theparameters are picked to minimize the average user decoding delay. Theattractive aspect of the whole operation is that, even though usertransmissions and optimistic rates are inherently coupled through thescheduler/MIMO transmission, the optimization of the ARQ modules can beperformed independently for each module and thus can be set and changedindependently to match the individual user's traffic/application demandsand constraints.

Examples of ARQ Parameters at UT “m”

The following provides an example of ARQ parameters used in the ARQblock generation process performed by the ARQ block generation unit. Inone embodiment, well-known models are used for setting the UT-ARQparameters. For example, a model may be used that is described in Caireet al., “The Throughput of Hybrid-{ARQ} Protocols for the GaussianCollision Channel”, IEEE Trans. On Inform. Theory, vol. 47, no. 5, pp1971-1988, July 2001, which is incorporated herein by reference. AHybrid-ARQ IR (incremental redundancy) block generation process that canbe used is described below. In this case, from each information packetof bits, the scheme generates a set of coded samples, where the codedsamples from a single packet are split into k_(max) segments (ARQblocks); these ARQ blocks are sequentially transmitted (preferably oneblock per packet per scheduling event), and sequential transmissionterminates once an ACK is received, or if k_(max) NACKs in a row arereceived (outage). Assuming UT “m” (long-term) average achievable rate(over scheduled slots) equals 0.5 bits per channel use, instantaneouslyachievable rate for UT “m” varies over slots, e.g., between 0.01 bits &4 bits per channel use; and 100 coded-ARQ samples are sent pertransmission.

The coding & modulation (C&M) parameters produce k_(max)×100 complexsamples that are sent over k_(max) transmissions, and information bitsare passed through a rate-Rc binary code, followed by an interleaverfollowed by mapper to Q-QAM to produce k_(max)×100 complex samples forthe k_(max) 100-sample transmissions.

Consider 3 C&M cases, all giving effective rate 0.4 bits/channel useafter a total of k_(max) TXs: QPSK (4-QAM) with Rc=⅕; 16-QAM with Rc=1/10; and 64-QAM with Rc= 1/15.

Tables below show for various k_(max) values: the 1st block coding rate(“R1”); k_(min) (the index of the received ARQ block after which the UTcan start decoding (before that it's all “NACK”s), and k_(avg) (thesmallest number of transmissions required for the coding rate to becomeequal to or fall below the average achievable rate). Each packet ofinformation bits has 100 R1 bits.

QPSK, R_(c) = 1/5 k_(max) 5 10 15 20 R₁ 2 4 6 8 k_(min) 1 2 3 4 k_(avg)4 8 12 16

16-QAM, R_(c) = 1/10 k_(max) 5 10 15 20 R₁ 2 4 6 8 k_(min) 1 1 2 2k_(avg) 4 8 12 16

64-QAM, R_(c) = 1/15 k_(max) 5 10 15 20 R₁ 2 4 6 8 k_(min) 1 1 1 1k_(avg) 4 8 12 16

Note that each of the blocks that are produced as part of the ARQ blockgeneration process represent different coded versions of the packet andthese coded blocks are sent across different scheduling instances. Thatis, the scheduler schedules one of the k ARQ blocks associated with thepacket at one scheduling instance and any other ARQ block associatedwith the same information packet is sent over a different schedulinginstance. If there are additional ARQ blocks to be sent for differentpackets, they may be sent in the same scheduling instance as an ARQblock for another packet as long as there is time available in thetransmission slot. Alternatively, one could decide that for a given userterminal “m” within each scheduling slot “K_(m)” information packetswould be served. In that case the transmission in each scheduling slotwhere user terminal “m” gets served would be split into “K_(m)” chunks,each chunk serving a different information packet (i.e, the kth chuckserving the kth packet then the kth chunk would be used to transmit thenext ARQ-block in the queue for the kth information packet).]

Scheduler State Update

In response to scheduling feedback 301 and additional information 302,which may include input buffer sizes, user terminal QOS constraints,fairness criteria, etc., the scheduling weight selection algorithm 300executed by processing logic of the base station generates updated userterminal scheduling weights 303 in a manner well known in the art.

In one embodiment, the scheduling feedback represents low rate feedbackinformation because it is sent by the user terminals every time the userterminal is scheduled and is sent at the end of the transmission slotbecause it is going to be used in the next transmission or in the nextscheduling instance to decide whether the user terminal can be scheduledor not. It could also be aggregated and updated every several schedulingcycles. Alternatively a coarser version can be fed back. As an example,PFS may be used where the scheduler weights are the inverses of theuser's time-averaged long term optimistic rates. In that case, at theend of the scheduling cycle, a scheduled user terminal need not needhave to feed the optimistic user rate. Instead, it can locally computeits time-averaged optimistic rate, differentially encode it and feed itto the base station so that the base station could compute itsscheduling weight. Such differential encoding of a slowly varying timeaveraged rate requires a much lower feedback bandwidth per userterminal.

Note that the input buffer sizes are utilized to determine whether thereis information available to send to the user terminal. In a case wherethere is no information to be sent to the user terminal, their weightwould be zero, such that they would not be selected for transmissionduring the next transmission slot.

The delay or fairness constraints may be used such as in a PFSalgorithm. In one embodiment, where the weights for all users are thesame, and the scheduler is PFS, the PFS operates equivalently (in termsof which users it schedules and the relative rate to such users) as asystem where each user gets its maximum (assumed) instantaneousUT-specific throughput. Delay or fairness constraints and their use inscheduling are well known in the art.

In one embodiment, the updated UT scheduling weights 303 are input intoa joint scheduler/precoder for scheduling in the next schedulinginstance/cycle.

Scheduler/Precoder Operation

In one embodiment, the scheduling algorithm (e.g., a joint ZFBF+PFSalgorithm) is run by each scheduler/precoder of the base stationindependently in order to schedule users and transmissions within eachscheduling instance. The scheduler/precoder may be part of a clustercontroller in the base station.

The scheduling algorithm receives the UT channel estimates (between thetransmit antennas and UTs) for the given resource block, statisticalinformation about unknown rate relevant parameters (e.g., the ICI levelexperienced by the UT (e.g., the UT average ICI) in the resource block),and scheduling parameters (e.g., PFS user weights for PFS) as inputs.

The scheduler/precoder executes the selection algorithm to decide how tobias what user terminal gets scheduled using the scheduling weights. Ifall of the scheduling weights are the same, all user terminals arebiased in the same manner and the selection of user that achieves thehighest sum rate are selected for transmission. If the schedulingweights are different, such as, for example, user terminal “A” has aweight twice as large as user terminal “B”, for the user terminal withthe smaller weight to be scheduled (i.e., terminal “A”), its predicteddelivered rate must be at least twice as large as the predicteddeliverer rate to user terminal “B”. Furthermore, in multi-user MIMO,where the weighted sum-rates of multiple users are considered at a time,then the selection and scheduling occurs jointly with 2 or more userterminals simultaneously being served and their weighted sum ratedetermines what group of user terminals are scheduled.

FIG. 4 illustrates a joint scheduler/precoder 400. In one embodiment,the joint scheduler/precoder comprises a greedy LZBF, which is wellknown in the art. Joint scheduler/precoder 400 receives UT schedulingweights 303 from the scheduler state update module running thescheduling weight selection algorithm 300, along with channel estimatesof the user terminals considered for scheduling 401. In response tothese inputs, joint scheduler/precoder 400 generates indices of userterminals selected for scheduling 402 and a user-specific precoder beamvector and its power for each scheduled user terminal 403. Thus thejoint scheduler/precoder 400 selects user terminals, specifies whichuser terminals are selected, and it also decides at what power and onwhat beam to put each transmission.

The indices of the user terminals selected for scheduling 402 and theprecoder beams and their powers are input to the ARQ signaling/MIMOtransmission module (403).

In one embodiment, the scheduler uses a “Greedy ZFBF PFS” algorithm asspecified in Caire et al., “Multiuser {MIMO} downlink with limitedinter-cell cooperation: Approximate interference alignment in time,frequency and space”, in Proc. 46^(th) Allerton Conf. Commun, Controland Computing, Monticello, Ill., October 2008, and Ramprashad et al.,“Cellular vs. network MIMO: A comparison including the channel stateinformation overhead”, in Proc. IEEE Intern. Symp. On Personal, Indoorand Mobile Radio Commun. PIMRC '07, Tokyo, Japan, September 2009, whichis incorporated herein by reference. However, note that there are manywell-known scheduling/MU-MIMO algorithms that can be used.

ARQ Blocks for Selection Process for a User Terminal

The ARQ signaling/MIMO transmission module also receives one or more ARQblocks that are selected for transmission to the user terminal.Similarly, ARQ blocks selected for transmission to other user terminalsare also input to respective ARQ signaling/MIMO transmission modules.

FIG. 5 illustrates one embodiment of an ARQ block selection process. Theprocess is performed by processing logic that may comprise hardware,software, or both.

In FIG. 5, the state variable n is the index for the information packetcurrently being transmitted by sample ARQ-process, the state variable kis the index of the ARQ block of the information packet “n” pointed toby given ARQ process, and k_(max) is the maximum number of ARQ blocksgenerated per packet. It is assumed that (n,k) equals n₁,k₁ for someintegers n₁ and k₁.

Referring to FIG. 5, using the scheduler's user terminal selection,processing logic determines whether the user terminal is scheduled(processing block 501). If not, the ARQ block (n, k) is equal to (n₁,k₁). If the user terminal is scheduled, the processes transmit the ARQblock (n, k) and the process transitions to processing block 502 whereprocessing logic determines if an ACK or NACK was received based onfeedback from the user terminal. If an ACK was received, processinglogic selects the next block for the next packet (n₂, k₂) fortransmission. If a NACK is received as the feedback, processing logictransitions to processing block 503 where processing logic determineswhether the index of the ARQ block for the information packet is greaterthan the maximum number of the ARQ blocks generated for the packet. Ifnot, the block that is selected is the next ARQ block for the packetcurrently being transmitted. If the index of the ARQ block is greaterthan the maximum number of ARQ blocks generated per packet, the currentpacket is in outage and the ARQ block selected is the next ARQ block forthe next packet (n₂,k₂). The values of (n₂,k₂) depend on whether asingle ARQ process (for a given scheduled user) is served within onescheduling slot, or whether multiple ARQ processes (serving distinctinformation packets) are served per slot.

For example:

-   -   Single ARQ Process: k₂=1, n₂=n₁+1 (start transmission of ARQ        blocks for the next information packet). In this case, when        serving the current packet “n” terminates (either because of an        “ACK” or because all k_(max) ARQ blocks have been exhausted for        the “n₁” packet, the process moves to serving the next packet        (n₁+1), starting from the first ARQ-block (k₂=1).    -   Multiple parallel ARQ processes: k₂=1, n₂ is the index of the        information packet with the smallest index (>n₁) that has not        yet been associated with (served by) a parallel ARQ process.

ARQ Signaling/MIMO Transmission Module

FIG. 6 illustrates the ARQ/PHY transmitter operation at the basestation. Referring to FIG. 6, the ARQ blocks for each user terminal aregenerated independently of the precoder/scheduling operation. Thus thek₁-th coded ARQ block from information packet “n₁” for user terminal“m₁” is generated independently of the k₂-th coded ARQ block forinformation packet “n₂” for user terminal “m₂” as well as theinstantaneous user channels over which these packets will be transmittedand the instantaneous precoders used to transmit these ARQ blocks. Eachof the ARQ blocks is input into a precoder that generates precoded datain response to the precoding beam for the user terminal that is outputfrom the scheduler/precoder, such as the scheduler/precoder shown inFIG. 4. The precoded data is input into a summer 603 which produces asummed output in a manner well known in the art. The summed output isinput into OFDM/RF block 604 which applies OFDM to the summed output ofsummer 603 and transmits the data as part of a MIMO transmission over anOFDM subband in a manner well known in the art. Although not shown inthe figure, this operation is typically performed in parallel over manydifferent OFDM subbands, each serving/scheduling a subset of userterminals.

This module may be part of a cluster controller for use in a wirelesscommunication system, such as MU-MIMO. The cluster controller controls acluster of transmission antennas. Such a controller may, for example,reside in (or be synonymous with) a base-station. In one embodiment, thecluster controller includes a scheduler/precoder, an ARQ blockgeneration mechanism, and Linear Zero Forced Beamforming (LZFB) for theMU-MIMO method being used to transmit the ARQ blocks.

Processing at the User Terminal

During its operation, the user terminal accumulates ARQ blocks in an ARQbuffer and determines whether it has received enough data to decode theinformation packet. When enough data has been received to decode thepacket, the user terminal sends an ACK signal to the base station. Inresponse to the ACK signal, the base station will clear any other ARQblocks for the information packet that has been prepared and stored,such that no additional ARQ blocks for the information packet will besent. The decoder sends a NACK to the base station to indicate to thebase station that it does not have enough ARQ blocks to decode theinformation packet and that at least one additional ARQ block for thepacket will have to be transmitted to the user terminal by the basestation. This enables the base station to know to send the ARQ blockthat is next in the sequential order.

The user terminal also processes pilot samples that do not carry dataand are used to estimate the channel of the user terminal. The channelestimate is used by the user terminal to determine the achievable rate,which forms part of the low rate scheduling feedback information that issent by the end of the scheduling cycle and used by the base station toupdate the scheduler state (for use as part of the scheduling weightselection algorithm of the base station by which the base stationselects user terminals for scheduling). This channel estimate enablesthe user terminal to estimate the best possible rate that could beachieved during the next scheduling instance. This best-possible rateestimate is based on also accounting for the instantaneous aggregateinterference level experienced by the user terminal due to thetransmissions of other base stations and/or other sources ofinterference that would not be known to the base station a priori.

FIG. 7 illustrates the processing that occurs at a user terminal, suchas user terminal “m”. Referring to FIG. 7, samples are received inreceive buffer 701. In one embodiment, the samples in receive buffer 701are complex-valued time-frequency samples. Out of all these receivedcomplex-valued time-frequency samples in receive buffer 701, one subsetof samples, pilot samples 702, carries pilot data for channelestimation, while another subset of samples contains all thedata-carrying transmission symbols (e.g., channel and noise corruptedversions of the samples transmitted by the base station (e.g., encodedsamples comprising the k-th ARQ block)). In one embodiment, the samplesin receive buffer 701 are created as a result of processing a receivedsignal waveform over an associated time-segment by an OFDM front-end,thereby resulting in the set of received complex-valued samples, one pertime-frequency slot of the many time-frequency slot-samples.

Pilot samples 702 are sent to channel estimator 703, which estimates theeffective channel of the user terminal “m” and generates a channelestimate 704 that is sent to achievable rate computation block 705 andblock 706. In response to channel estimate 704, achievable ratecomputation block 705 determines the achievable rate for the userterminal in view of the channel estimate 704. In one embodiment, this isdetermined in a manner well known in the art by computing it as thebase-2 logarithm of “1” plus the instantaneous SINR (signal tointerference-plus-noise ratio). The achievable rate computation formsthe low rate scheduling feedback 707 that is fed back to the basestation at the end of the scheduling cycle. That is, this achievablerate information is fed back at the end of the current transmissioncycle such that it may be used as part of the next scheduling cycle todetermine whether the user terminal “m” is to be scheduled or not. Asdescribed earlier, low scheduling feedback-rate implementations are alsopossible.

The receiver takes the samples in this second subset (together with thechannel estimates it formed for the k-th block based on the first subsetof samples) and also takes all the previous blocks' received samples(and their channel estimates) which are stored in memory 708, and thenperforms decoding based on the totality of the received samples in ARQblocks 1 through k, taking into account the individual channel estimatesin blocks 1 through k (along with their individual quality) that varyfrom block to block. More specifically, the received samples receivedinto receive buffer 701 for the “k”-th ARQ block for packet “n” for userterminal “m” are input into block 706 that also receives the channelestimate 704 and previously received ARQ blocks for packet “n”, alongwith the channel estimates that were generated at the time thoseprevious ARQ blocks were received. These were stored in ARQ buffer 708at the time of the earlier transmissions. In response to these inputs,block 706 constructs a large vector comprising of the concatenation ofthe “k” received sample blocks, along with the associated “k” receivedsignal SINRs (or along with the associated measured “k” {signal power,interference-plus-noise power} level pairs) and these are sent todecoder 710. In response to all the “k” ARQ blocks and channelestimates, decoder 710 determines whether it can decode the informationpacket from the ARQ blocks received. This occurs in a manner well knownin the art. If it can, decoder 710 decodes the data and sends the ACKback to the base station and uses the Clear ARQ buffer signal to clearits own ARQ buffer (708). If not, decoder sends a NACK feedback signalto the base station and stores the “k”-th ARQ block and the channelestimate for that “k”-th ARQ block into ARQ buffer 708.

An Example of a User Terminal

FIG. 8 is a block diagram of one embodiment of a user terminal (e.g.,mobile phone) that includes a transmitter and/or the receiver andincludes components described above (e.g., components of FIG. 7).

Referring to FIG. 8, the cellular phone 810 includes an antenna 811, aradio-frequency transceiver (an RF unit) 812, a modem 813, a signalprocessing unit 814, a control unit 815, an external interface unit(external I/F) 816, a speaker (SP) 817, a microphone (MIC) 818, adisplay unit 819, an operation unit 820 and a memory 821.

In one embodiment, the external I/F 816 includes an external interface(external I/F), a CPU (Central Processing Unit), a display unit, akeyboard, a memory, a hard disk and a CD-ROM drive.

The CPU and the control unit 815 in cooperation with the memories ofcellular phone 810 (e.g., memory 821, memory, and hard disk of theexternal I/F 816) cooperate to perform the operations described above.

Note that the transmitter and/or receiver may be included in otherwireless devices (e.g., a wireless LAN).

The external I/F can be connected to a notebook, laptop, desktop orother computer. This can enable the user terminal to act as a wirelessmodem for the computer. The user terminal can be the computer'sconnection to the internet, WiFi and WiMAX, a local area network, a widearea network, a personal area network, Bluetooth.

An Example of a Computer System

FIG. 9 is a block diagram of an exemplary computer system that mayperform one or more of the operations described herein. For example,such a computer system or parts thereof could run the model describedabove.

Referring to FIG. 9, computer system 900 may comprise an exemplaryclient or server computer system. Computer system 900 comprises acommunication mechanism or bus 911 for communicating information, and aprocessor 912 coupled with bus 911 for processing information. Processor912 includes a microprocessor, but is not limited to a microprocessor,such as, for example, Pentium™, PowerPC™, Alpha™, etc.

System 900 further comprises a random access memory (RAM), or otherdynamic storage device 904 (referred to as main memory) coupled to bus911 for storing information and instructions to be executed by processor912. Main memory 904 also may be used for storing temporary variables orother intermediate information during execution of instructions byprocessor 912.

Computer system 900 also comprises a read only memory (ROM) and/or otherstatic storage device 906 coupled to bus 911 for storing staticinformation and instructions for processor 912, and a data storagedevice 907, such as a magnetic disk or optical disk and itscorresponding disk drive. Data storage device 907 is coupled to bus 911for storing information and instructions.

Computer system 900 may further be coupled to a display device 921, suchas a cathode ray tube (CRT) or liquid crystal display (LCD), coupled tobus 911 for displaying information to a computer user. An alphanumericinput device 922, including alphanumeric and other keys, may also becoupled to bus 911 for communicating information and command selectionsto processor 912. An additional user input device is cursor control 923,such as a mouse, trackball, trackpad, stylus, or cursor direction keys,coupled to bus 911 for communicating direction information and commandselections to processor 912, and for controlling cursor movement ondisplay 921.

Another device that may be coupled to bus 911 is hard copy device 924,which may be used for marking information on a medium such as paper,film, or similar types of media. Another device that may be coupled tobus 911 is a wired/wireless communication capability 925 tocommunication to a phone or handheld palm device.

Note that any or all of the components of system 900 and associatedhardware may be used in the present invention. However, it can beappreciated that other configurations of the computer system may includesome or all of the devices.

Whereas many alterations and modifications of the present invention willno doubt become apparent to a person of ordinary skill in the art afterhaving read the foregoing description, it is to be understood that anyparticular embodiment shown and described by way of illustration is inno way intended to be considered limiting. Therefore, references todetails of various embodiments are not intended to limit the scope ofthe claims which in themselves recite only those features regarded asessential to the invention.

1. A method comprising: storing received samples and channel estimatesfrom previous ARQ transmissions in an ARQ buffer; receiving samples in areceive buffer; estimating an effective channel of a user terminal basedon received pilot samples from the receive buffer; computing theachievable rate based on the channel estimate and feeding backinformation on the achievable rate by an end of a current schedulingcycle; determining whether the packet can be decoded based on k receivedARQ blocks for the packet, wherein the k received ARQ blocks comprisesamples for the k-th ARQ block for the packet from the receive bufferand samples for previously received k−1 ARQ blocks for the packet, wherek is an integer equal to 1 or greater; decoding the packet and sendingback an acknowledgement (ACK) signal as feedback if determining thepacket is decodable based on the k ARQ blocks; and storing the k-th ARQblock in the ARQ buffer and sending a negative acknowledgement (NACK)signal as feedback if determining the packet is not decodable based onthe k ARQ blocks.
 2. The method defined in claim 1 wherein the pluralityof ARQ blocks were generated independently of scheduling and precoding.3. The method defined in claim 1 wherein generating the plurality of ARQblocks were generated using a set of parameters associated with the oneuser terminal, and wherein the set of parameters are independent ofknowledge of the instantaneous channel.
 4. The method defined in claim 1wherein generating the plurality of ARQ blocks were generated by:receiving information bits for the one user terminal; splitting thereceived information bits into a plurality of packets based on an ARQblock coding rate; and applying an ARQ block generator process to eachof the plurality of packets based on a signaled ARQ scheme, and codingand modulation parameters to produce a plurality of ARQ blocks for eachof the plurality of packets, including the plurality of ARQ blocks forthe single packet, wherein each of the ARQ blocks comprises codedsamples.
 5. The method defined in claim 1 wherein the ARQ blocks for thepacket are scheduled and transmitted over distinct transmission slots(or over transmission slots corresponding to distinct schedulingevents).
 6. The method defined in claim 1 further comprising selectingone ARQ block for transmission by: determining whether the one userterminal is scheduled; if the one user terminal is scheduled,determining if feedback information from the one user terminal for aprevious one of the ARQ blocks for the packet is an acknowledgement(ACK) signal, selecting an ARQ block for another packet for transmissionif the feedback information is an ACK signal; selecting the one ARQblock for transmission if the feedback information is a negativeacknowledgement (NACK) signal, the plurality of ARQ blocks being asequence of ARQ blocks to be transmitted in sequential order, and theone ARQ block being the next in the sequential order the ARQ block to betransmitted.
 7. A method comprising: generating a plurality of ARQblocks for a single packet that is to be sent to one user terminal of aplurality of user terminals; updating user terminal scheduling weightsin response to scheduling feedback transmitted by the plurality of userterminals by an end of an immediately preceeding scheduling event;scheduling a set of the plurality of user terminals for a schedulingslot based on the updated user terminal scheduling weights; selectingprecoder beams and their power for each user terminal in the set;generating precoded data from one of the ARQ blocks using a selectedprecoded beam and its selected power selected for the one user terminalfor the scheduling slot; and transmitting the precoded data as part of aMIMO transmission process.
 8. The method defined in claim 7 wherein theplurality of ARQ blocks are generated independently of scheduling andprecoding.
 9. The method defined in claim 7 wherein generating theplurality of ARQ blocks is performed using a set of parametersassociated with the one user terminal, and wherein the set of parametersare independent of knowledge of the instantaneous channel.
 10. Themethod defined in claim 7 wherein generating the plurality of ARQ blockscomprises: receiving information bits for the one user terminal;splitting the received information bits into a plurality of packetsbased on an ARQ block coding rate; and applying an ARQ block generatorprocess to each of the plurality of packets based on a signaled ARQscheme, and coding and modulation parameters to produce a plurality ofARQ blocks for each of the plurality of packets, including the pluralityof ARQ blocks for the single packet, wherein each of the ARQ blockscomprises coded samples.
 11. The method defined in claim 7 whereintransmitting the precoded data occurs such that the ARQ blocks for thepacket are scheduled and transmitted over distinct transmission slots(or over transmission slots corresponding to distinct schedulingevents).
 12. The method defined in claim 7 further comprising selectingthe one ARQ block for transmission by: determining whether the one userterminal is scheduled; if the one user terminal is scheduled,determining if feedback information from the one user terminal for aprevious one of the ARQ blocks for the packet is an acknowledgement(ACK) signal, selecting an ARQ block for another packet for transmissionif the feedback information is an ACK signal; selecting the one ARQblock for transmission if the feedback information is a negativeacknowledgement (NACK) signal, the plurality of ARQ blocks being asequence of ARQ blocks to be transmitted in sequential order, and theone ARQ block being the next in the sequential order the ARQ block to betransmitted.
 13. A product having one or more computer readable storagemedia storing executable instructions thereon which when executed causea user terminal to perform a method, the method comprising: storingreceived samples and channel estimates from previous ARQ transmissionsin an ARQ buffer; receiving samples in a receive buffer; estimating aneffective channel of the user terminal based on received pilot samplesfrom the receive buffer; computing the achievable rate based on thechannel estimate and feeding back information on the achievable rate byan end of a current scheduling cycle; generating k ARQ blocks fromreceived samples for the k-th ARQ block for a packet and previouslyreceived k−1 ARQ blocks for the packet, where k is an integer equal to 1or greater; determining whether the packet can be decoded based on the kARQ blocks; decoding the packet and sending back an acknowledgement(ACK) signal as feedback if determining the packet is decodable based onthe k ARQ blocks; and storing the k-th ARQ block in the ARQ buffer andsending a negative acknowledgement (NACK) signal as feedback ifdetermining the packet is not decodable based on the k ARQ blocks.